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docs: Add example of reordering stacked bars #3395

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Apr 12, 2024
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docs: Add example of reordering stacked bars #3395

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Add example of reordering stacked bars
joelostblom Apr 6, 2024
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joelostblom Apr 8, 2024
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30 changes: 30 additions & 0 deletions tests/examples_arguments_syntax/interactive_reorder_stacked_bars.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,30 @@
"""
Reorder stacked bar segments
============================
This example uses a calculate transform
to check the values of the "site" column
vs the clicked values in the legend,
and assigns a lower order (0)
if there is a match.
The use of "indexOf" checks for equality in an array,
which here allows for mutliple segments to be reordered
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by holding down the shift key while clicking the legend.
"""
# category: interactive charts
import altair as alt
from vega_datasets import data


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selection = alt.selection_point(fields=['site'], bind='legend')

alt.Chart(data.barley.url).mark_bar().transform_calculate(
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site_order=f"if({selection.name}.site && indexof({selection.name}.site, datum.site) !== -1, 0, 1)"
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Im fine with current approach, but an observation is that this is JavaScript syntax. I'm not sure if the Python syntax for this type of Vega expressions is more readable.

While often underestimated in efficiency, we don't really have a good approach documented to work with these Vega expressions.

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Yes, @jonmmease made a similar comment in #3362 (comment) and I would also prefer to change to the Python syntax in general in the docs. However, I think we should solve #3366 first, so that the Py syntax covers everything that the JS syntax can do (this instance is the only difference I'm aware of).

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Ah, yes, I remember being bitten by this recently too!

).encode(
x='sum(yield):Q',
y='variety:N',
color='site:N',
order='site_order:N',
opacity=alt.condition(selection, alt.value(0.9), alt.value(0.2))
).add_params(
selection
)
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